1. Field of the Invention The present invention relates to a method and a system using the method for monitoring the quality of fluids, comprising the measurement of different features of a fluid with at least two different methods and deriving a measure for the quality of the fluid and/or identifying a process of the fluid deterioration by comparing the results of the measurements of the at least two different methods.
2. Description of the Related Art The quality monitoring of fluids is particularly important for defect free, continuous operation of transformers, switchgears, fuses and other equipment, such as in power grids. Among other things, fluids are used as transformer oil or dielectric fluid in electrical equipment, and are usually highly-refined mineral oils or synthetic esters, which are stable at high temperatures and have excellent electrical insulating properties. The quality of the fluids plays an important role for the proper function of the equipment comprising the fluid. The quality changes due to changes in properties of these fluids, such as acidity, water content or the concentration of free radicals. The natural degradation can be accelerated by elevated temperature and/or pressure, or by chemical reactions with other components, such as like plastic parts.
The qualities of the fluids and degradation have to be monitored to identify and forecast the ageing process of used fluids and the entire apparatus/construction. The fluids, particularly liquids, are used to insulate, suppress corona and arcing, and to serve as a coolant, such as in oil-filled transformers, some types of high voltage capacitors, fluorescent lamp ballasts, and sortie types of high voltage switches and circuit breakers. A proper function of a used fluid and of the apparatus is only assured with a certain level of fluid quality. In subsea applications the quality has to be remotely monitored.
A low fluid quality can lead to a breakdown of an apparatus and thereby destroy equipment. To prevent this, fluids have to be replaced when they degrade below a certain, predefined quality level. The degradation below the quality level is identified by monitoring the fluid, particularly liquid. A continuous monitoring reduces the time to replace degraded fluids and the risk of failure.
A sensing technique for the monitoring of a fluid has to be reliable, miniaturized, insensitive to electromagnetic noise, and in some oases able to withstand harsh environments, i.e., high pressure of up to 300 bar, such as in subsea applications and high temperature.
From the 1990s, on-line monitoring particularly of transformers has become increasingly popular tending to reduce the number of time-based diagnostic operations, where in some cases equipment has to be shut down. The most common methods known from the state of the art, both monitoring and diagnostics methods are:                Dissolved gas analysis (DGA):                    Multiple gas sensor;            Hydrogen sensor;            Individual gas sensors;                        Partial discharge (PD) monitoring:                    Glass fiber rods;            Electrical (RF coils);            Electrical (phase impulse current);            Acoustic;            Via DGA (Hydrogen);                        Cellulose and oil moisture content:                    Dielectric response analysis;            Capacitive probes;            Fiber optical;            Karl Fischer titration;                        Degree of polymerization (DP):                    Paper samples;            Furanic compounds analysis;                        Spectroscopy/Transparency:                    Ultraviolet/visible—UV/VIS;            NIR;            MIR;            Integral attenuation in a broad spectrum range;            Visual evaluation;                        Acidity tests;        Dielectric strength tests;        Resistivity tests.        
DGA utilizes the measurement of concentrations of H2, CH4, C2H6, C2H4, C2H2/CO and CO2 for transformer fault detection. With respect to remote condition monitoring, DGA based systems implement either gas chromatography or photo-acoustic spectroscopy. Both techniques require dissolved gas separation from oil, which is not feasible under high pressure.
PD activity monitoring is a convenient tool to detect degradation of the transformer insulation. Nevertheless, it does not provide any information on dielectric fluid contaminants composition.
Karl Fischer titration, DP measurement with paper samples, acidity test and dielectric strength test are not feasible for in-situ implementation. Dielectric response analysis requires transformer shut-down. Capacitive moisture sensors withstanding pressures up to 200 bar are known, but this is often not sufficient to meet requirements.
MIR spectroscopy is a standardized laboratory technique for inspecting insulating oil but it is not specialized for aging mechanism determination. The technique is more appropriate for lubricants.
Separately, none of the techniques can provide enough data for fluid condition monitoring and fluid aging mechanism determination. Many of the above-listed technologies are intended for laboratory use only.